hello-world: Modify tests to reject `name == ""` comparison (#280)

* hello-world: Modify tests to reject name == "" comparison

* hello-world: Add emptyStringIsComparedByValue() test case

exercises/hello-world/src/test/java/HelloWorldTest.java

 
     @Test
     @Ignore
+    public void emptyStringIsComparedByValue() {
+        assertEquals("Hello, World!", HelloWorld.hello(new String("")));
+    }
+    
+    @Test
+    @Ignore
     public void helloSampleName() {
         assertEquals("Hello, Alice!", HelloWorld.hello("Alice"));
     }